In radiation therapy, accurate irradiation of the affected region (e.g., a tumor) is important. Meanwhile, a living organ moves in various manners, and a tumor positioned near a lung, in particular, moves largely based on respiration. Therefore, it has been proposed, with a marker placed near a tumor, to determine the position of the marker, using a bi-directional X-ray fluoroscopic moving object tracking device to control irradiation of therapy radiation (Patent Document 1). This moving object tracking device captures an image of a metallic marker or the like embedded in a body, using two sets of high-power X-ray producing device and X-ray image receiving devices, and measures the 3D position, based on two 2D fluoroscopic images. Successive capturing of X-ray fluoroscopic images enables real-time calculation of the 3D position of a metallic marker, and therefore, a metallic marker in a position subjected to respiration-caused movement can be detected with high accuracy. In view of the above, it is possible to irradiate radiation with high accuracy according to the movement of a tumor by controlling irradiation of therapy radiation, based on a result of detection of a metallic marker.